Tropical update: Lesser Antilles threat

ATLANTA, GA, U.S.A. (TWCC; 05:40AM EDT) -- Thunderstorms have increased some near the circulation of 93-L. The environment ahead remains only marginally favorable for development but just a slight uptick in thunderstorms could kick off a tropical depression.

Thunderstorms have increased some near the circulation of 93-L. The environment ahead remains only marginally favorable for development but just a slight uptick in thunderstorms could kick off a tropical depression.

The disturbance, shown at the center right of the satellite image above, has a pronounced spin above the surface, but has yet to produce sufficiently persistent convection (translation: thunderstorms) and a pronounced-enough westerly surface wind to merit classification as a tropical cyclone.

Once those two events occur, the National Hurricane Center will likely begin issuing advisories on what would be Tropical Depression Three.

Forecast: Lesser Antilles This Weekend

First, the disturbance has to overcome some dry air, light wind shear and marginal sea-surface temperatures over the next day or so.

The National Hurricane Center has tentatively scheduled a Thursday afternoon flight into the disturbance, leaving from Christiansted, Henry E. Rohlsen Airport in St. Croix. A NOAA Gulfstream jet is also scheduled to sample the environment around the disturbance Thursday, in the hopes of improving model forecasts of both track and intensity.

Assuming a tropical depression does form, the system is expected to track toward the Lesser Antilles late Friday into the weekend, possibly strengthening into a tropical storm. Its name would be Bertha.

As the system nears the islands, an area of low pressure aloft (called a tropical upper tropospheric trough or TUTT) may begin to interact with potential future tropical cyclone. If Three/Bertha is stronger at that time, the TUTT would pull the system farther north. Conversely, a weaker, less vertically-developed system would not be pulled as far north by the TUTT.

Furthermore, this TUTT may produce wind shear (change in wind direction and/or speed with height) that is hostile for tropical cyclone intensification or development as the system nears the islands.

So, for now, interests in the Lesser Antilles, U.S. Virgin Islands and Puerto Rico should monitor the forecast closely for possible impacts this weekend, including at least an increase in showers and squalls.

It is still too soon to forecast any future potential impacts farther downstream, including in the Bahamas, the U.S. or Bermuda.

Incidentally, the average date by which the Atlantic hurricane season's second named storm will have occurred is Aug. 1, so this potential Bertha is right on time, climatologically.